The motor draws inductive current when a 50 HP induction motor is run by feeding a three-phase AC supply. At the no-load running of the motor major part of the current is inductive in nature and called magnetizing current. When a motor is run on load, the motor draws an inductive current.
The inductive current drawn by the motor increases the heat loss in the cable and in the motor winding. To reduce the line current, the capacitor is connected across the motor winding to compensate for the reactive current. Required capacitor KVAR for 50 Hp motor to compensate inductive current can be done as follows.
First, calculate the reactive KVAR of the motor operating under no-load.
KVAR of the Motor =√3 VL IL Sinθ
VL and IL are the line voltage and the line current, respectively. The angle θ is the phase angle between voltage and current.
The line current is the vector sum of the current drawn by the capacitor and motor.
Required capacitor KVAR for 50 HP motor to nullify the effect of inductive KVAR of the motor must be just close to inductive KVAr of the motor.
50 HP = 0.745 x 50 = 37 KW
The motor draws about 1/3 rd of its Full Load Current under no-load conditions. The capacitive KVAR required to compensate magnetizing current of the motor under no-load conditions is;
Required Capacitor KVAR for 50 HP Motor
=1/ 3 * Motor rated KW
= 1/3 * 37
=12 KVAR
If installed capacitive KVAR is higher than the running inductive KVAR, this situation may lead to a leading power factor that will increase the line voltage. The increased voltage may break down the motor insulation.
